tablet always on charge, big or low (car)battary drain??

hello guys. i start this thread, becouse, no matter how i research, i cant find a specific answer, to as "the tablet will drain the battery?".
so, my situtation is like this:
i will use a tablet as a speedometer,so it has to be completly automated, and responsive.the thing is, i drive my car, only like once a week, and not for long drives, so even IF the tablet would last a week on sleep, it wont fully charge when i drive the car...

My fist thought, was to use an automated system (arduino) to auto charge the tablet, like every 2 days, but in that way, i cant use the tasker app. to auto turn on the tablet..

so i thought, to do the opposite. have the tablet always on charge, with a switching psu, and when i turn off the car, REMOVE the tablet from charge. that way i can have the tablet always charged, and also use tasker to auto turn it on and off.

so the concerns would be those: (Asuming you have a second battery, isolated, and AGREE that batteries are meant to be discharged (otherwise they would not be called batteries... )

1)what would be the effect of continious charge , in the lithium-ion batterie?
as i see, it want have a probelm.. tablets and laptops ,have sofisticated circuits to control their charge, so, overcharge , is not an issue!! ( ??)
2) what would be the drain in the second batterie ??
from what i see, after the tablet fully charges, it only needs as power, as it consumes. (??? ) so, the real question is how much power the tablet uses in deep sleep...
using an app (current widget) , you can make a graph of the "actual" mA consumption of the tablet. when it is on, it varies from 400mA to 1000mA BUT , when the tablet sleeps it goes from as low to 1mA(?!?!) to 30mA... wich to me seem extremly small to be true... but if that is the case, then the tablet(withe the dc-dc converter) , will never draw more than 100mA , and the car batterie , would last for weeks, without a problem....